At present, horizontal hydraulic jacks commonly seen in the market mainly comprise a chassis, a hydraulic assembly, a boom assembly, a link rod, and a handle lever, wherein, the boom assembly comprises a boom and a bracket arranged on the boom, one end of the link rod is pivotally jointed to the chassis, and the other end of the link rod is pivotally jointed to the bracket of the boom assembly. In the lifting process, as the handle lever is pressed repeatedly, the hydraulic assembly drives the boom assembly to lift up, and finally the bracket on the boom jacks up the heavy load. The structure described above has the following drawback: for a horizontal hydraulic jack with same specification, i.e., the parameters such as oil pressure and stroke are specific, both the lifting height and the load capacity are fixed and can not be changed because both the boom length and the link rod length can not be adjusted; therefore, such a horizontal hydraulic jack is very simple in functionality and has limited applicability. To solve the above problems, a hydraulic jack in which the height and load at the lowest operating position can be changed effectively by adjusting the boom length and the link rod length under the condition of equivalent parameters. For example, in Chinese Invention Patent Publication No. CN101691194B, an “adjustable horizontal hydraulic jack” is disclosed, in which the transition between different states of boom position is implemented with a fixing pin; specifically, when the fixing pin is used to fix the bracket and fore arm, the boom and the link rod are in a folded position state; whereas, when the fixing pin is used to fix the fore arm and rear arm, the boom and the link rod are in normal position state. However, utilizing a fixing pin to change the position state of the boom is complicated in operation, and has potential safety risks in actual application. For another example, in Chinese Utility Model Patent Publication No. CN202358873U, a “Boom-Adjustable Horizontal Hydraulic Jack with a Pull Lock Mechanism” is disclosed, in which the transition between different states of boom position is implemented with a pull lock mechanism; however, the pull lock mechanism is mounted on the bracket and they are at the same elevation; consequently, when the boom jacks up, it may hit on the pull lock mechanism easily if the operation is inappropriate, causing damage to the pull lock mechanism and degraded service life of the lifting jack. In addition, such a structure is high in fabrication cost and difficult to assemble.